Pump and compressor designers use engineering principles to design a wide variety of pumps and compressors to meet industrial performance requirements. The design practice of the past century has been based principally on the assumptions of axisymmetric steady flow through turbomachines with most emphasis placed on the principal through-flow or primary passage flow. This historical design work has often encountered limitations due to stability problems as the flow is reduced and the head or pressure rises. Under this operation, the incidence on every vane element in the turbomachinery increases, levels of diffusion along critical surfaces rise substantially, and levels of secondary flow within passages increase.
Secondary fluid flows are elements of the overall flow field that have been subjected to force gradients across the main flow passage and result in vortices, skewed, i.e., three-dimensional, boundary layers, and so forth. Examples of secondary fluid flows are the well-known tip or part-span vortices, horseshoe vortices, passage vortices, backflow and recirculation flows, and other areas of flow which satisfy the laws of conservation of vorticity. These flows are much more difficult to work with and never contribute to improvement in performance of the stage. Yet, they cannot be avoided in a turbomachine, which always has cross-channel force gradients due to its fundamental nature, as reflected in the fundamental equations of motion covering these machines. All of these effects lead to the near certainty of stability problems of various types. Nearly all compressors experience a surge limit or boundary below which a machine cannot be operated without at least causing damage to the machine. Industrial practice uniformly rules out any operation below the surge line and even within a nominal percentage, i.e., typically five or ten percent, sometimes more, from this surge line. Designers have learned to respect this limit for compressors and an equivalent process for pumps. For pumps, surge usually does not occur as it does for compressors (due to lack of compliance in the system), but instabilities none-the-less occur which can destroy a pump or the entire system. An extreme example of pump instability is the Pogo effect. Many boiler feed pumps are limited by severe component stalling phenomena and there are resultant instabilities at part load.
It is desirable to have flexibility in shaping the head characteristic of a pump or a compressor. This may be accomplished by variable geometry elements such as variable inlet guide vanes, variable diffuser vanes, or equivalent devices. However, they are expensive, mechanically complex, and may reduce the operating time of the machine between maintenance intervals.
In addition to controlling secondary fluid flows, a further particular requirement for pumps is to provide the greatest possible suction capability before a particular breakdown phenomena known as cavitation occurs. Obviously, the first task for any pump or compressor is to create a low pressure at the inlet of the impeller so that fluid is drawn into the eye or inlet of that stage. Thus, it is well known that the lowest pressure point in a pump or compressor is usually very near the eye. Effects of blade blockage and incidence effects also cause local acceleration that can further drop the inlet static pressure. For the particular case of pumps, when this low inlet pressure drops below the vapor pressure of the liquid, bubbles are formed. These bubbles are referred to as cavitating flow. The bubbles are formed and then collapse later in the stage (unless there is too much cavitation that blocks the head rise characteristic of the impeller.) When the bubbles are collapsed, serious damage may occur and metal may be eroded away from the surface of even the toughest metallic vanes of a high-performance pump. This is a severe situation and one that must be designed for in all pump applications.
But the conditions may be even worse. In the process of setting up cavitation, certain instabilities occur from time to time. Critical aerospace applications and numerous industrial applications are limited in part, or in total, by the instability caused as cavitating flow switches into different locations in the downstream flow elements. This switching leads to an auto-oscillation that can cause enormous problems, such as the Pogo effect mentioned above. It is highly desirable to eliminate these instabilities. Additionally, the basic nature of the performance characteristic, as one approaches the breakdown point, must be dealt with. The conventional performance shows a progressive breakdown where the head is dropped as cavitation grows and blocks more and more of the passage with its vapor cavities. Indeed, even the standard design practice of remaining above 3% head breakdown does not eliminate the damage to the stage, but instead frequently assures that operation will occur in the region of greatest cavitation damage.
The prior art teaches or suggests the value of bleeding flow off at appropriate shroud line locations and either dumping the bleed overboard or reintroducing it somewhere upstream in order to improve flow capacity of a stage and to mitigate some of the effects spoken about above. However, most, if not all, prior art devices teach methods that are brutal to the flow and simply destroy or dissipate the energy that is bled off before the flow is allowed to be re-introduced.